There have long existed many methods for the application of a selected metal coating to a non-conductive, i.e., dielectric surface in order to produce printed circuit boards which will conduct an electrical current in accordance with the patterns of conductive metal coated on their surface. These methods have involved the following two basic steps: (1) treating the surface of the non-conductive substrate with an agent to make it catalytically receptive to electrolessly formed metal deposits; and (2) electrodepositing a plating metal over the electrolessly formed conductive metal deposits. The pattern of the printed circuit is achieved through the use of screen or photoresist imaging. The non-conductive substrate may initially be copper-clad or not; but most boards have copper cladding at the beginning of the process, which is later removed in the non-pattern areas. Such processes are, consequently, referred to as substractive.
In the typical processes relevant to printed circuit board manufacture, wherein through-hole metallization is employed, the catalytic material most often comprises palladium metal. The process of applying the catalytic material to the substrate surfaces typically involves contact of the substrate with a true or colloidal solution of palladium and tin compounds. See, e.g., U.S. Pat. Nos. 3,011,920 and 3,532,518. It is generally considered that the tin compounds act as protective colloids for the catalytic palladium. In most cases, the catalysis of the non-conductive substrate of the printed circuit board is followed by an "acceleration" step which exposes or increases exposure of the active catalytic species.
Following deposition of catalyst material on the non-conductive surfaces in the manner described, the surfaces are then contacted with an electroless metal depositing solution in which plating chemical reduction leads to the deposit of metal from the bath onto the catalyzed surface. The through-holes are usually plated with a copper reduction procedure known to the art as electroless copper plating, such as that described by Clyde F. Coombs, Jr. in Printed Circuit Handbook, 3rd Edition, McGraw-Hill Book Co., New York, N.Y., 1988, Chapter 12.5, which is incorporated herein by reference in its entirety.
Methods of the type described above, while apparently simple, have proven to be expensive and demanding of strict process controls. Further limitations on the use of these processes result from the chemical susceptibility of the electroless metal layer, and by the required use of very hazardous and toxic chemical agents. Efforts to overcome these disadvantages have met with only partial success in the past, and have brought with them their own disadvantages. Accordingly, in order to appreciate the significance of the improvements achieved by the present invention, it will be helpful to review beforehand the major features of current printed circuit board technology.
In a typical process for the manufacture of a single- or double-sided printed circuit board, suitable substrates generally comprise laminates consisting of two or more plates or foils of copper, which are separated from each other by a layer of non-conductive material. The non-conductive layer or layers are preferably an organic material such as epoxy resin impregnated with glass fibers. Holes are drilled or punched at appropriate locations on the board, providing side-to-side connections when metallized. Thereafter, the board is treated with a cleaning composition, typically alkaline, which removes soils and conditions the through-holes, followed by a slow acid etching treatment which is used for removal of copper surface pretreatments, oxidation, and presentation of uniformly active copper. Typical compositions for this microetching step are persulfates and sulfuric acid-hydrogen peroxide solutions. The board is next catalyzed with a neutral or acid solution of tin/palladium catalyst, which deposits a thin layer of surface-active palladium in the through-holes and on the surface of the board. Colloidal tin on the board surfaces and through-holes is removed by treatment with an accelerator composition. The board is then ready for electroless copper plating, which is typically carried out with an alkaline chelated copper reducing solution that deposits a thin copper layer in the through-holes and on the surfaces of the board. After acid-dipping, commonly with sulfuric acid, the board is metal plated with a conventional copper plating solution. It is more usual, however, to precede this metallization step with an imaging step.
In a process known as pattern plating, a dry film photoresist is applied to the board and then exposed to transfer the negative image of the circuit, after which it is developed to remove the unexposed portions. The resist coats the copper that is not part of the conductor pattern. Thickness of the copper pattern is increased by electrolytic copper plating. The imaged dry film resist is then removed, exposing unwanted copper, i.e., copper which is not part of the conductor pattern, and said unwanted copper is dissolved with a suitable etchant, e.g., ammoniacal copper or sulfuric/peroxide.
A multilayered printed circuit board is made by a similar process, except that pre-formed circuit boards are stacked on top of each other and coated with a dielectric layer. The stack is pressed and bonded together under heat and pressure, after which holes are drilled and plated in the above-described manner. However, one problem present with the manufacture of multilayer printed circuit board through-holes is that the drilling of the holes causes resin "smear" on the exposed conductive copper metal innerlayers, due to heating during the drilling operation. The resin smear may act as an insulator between the later plated-on metal in the through-holes and these copper innerlayers. Thus, this smear may result in poor electrical connections and must be removed before the plating-on operation.
Various alkaline permanganate treatments have been used as standard methods for desmearing surfaces of printed circuit boards, including the through-holes. Such permanganate treatments have been employed for reliably removing smear and drilling debris, as well as for texturing or micro-roughening the exposed epoxy resin surfaces. The latter effect significantly improves through-hole metallization by facilitating adhesion to epoxy resin. Other conventional smear removal methods have included treatment with sulfuric acid, chromic acid, and plasma desmear, which is a dry chemical method in which boards are exposed to oxygen and fluorocarbon gases, e.g., CF4. Generally, permanganate treatments involve three different solution treatments used sequentially. They are (1) a solvent swell solution, (2) a permanganate desmear solution, and (3) a neutralization solution. Typically, a printed circuit board is dipped or otherwise exposed to each solution, with a deionized water rinse between each of the three treatment solutions. When the desmearing process is continued, it results in exposure of about 0.5 mil on the top and bottom surface of the inner-layer copper, allowing it to protrude from the drilled through-hole, promoting better adhesion to the later-applied metallized layer.